The purpose of this study is to characterize baroreflexes arising from the left atrium and lung vessels which alter systemic vascular resistance and cardiac performance. Pressure within the combined system consisting of the left heart chambers and lung vessels of pump-perfused dogs will be sinusoidally forced at several frequencies and amplitudes under conditions of constant mean pulmonary blood flow, ventilation and systemic arterial pressure. Resultant changes in systemic vascular resistance will be used along with the forcing parameters to display the input-output relationships of the lumped cardio-pulmonary baroreflex system in terms of response magnitude and phase shift versus input frequency and amplitude. The variations in systemic vascular resistance obtained while forcing pressure selectively in the pulmonary arteries, veins and left atrium with both sinusoidal and step changes will be observed under similar conditions, and comparable data prepared. The interaction of changes in lung inflation conditions with the baroreflexes will be appraised by comparing responses evoked by uniform stimuli when ventilatory rate, tidal volume and end-expiratory pressure are varied in dogs pump-perfused as before, but in which gas exchange is achieved independent of lung ventilation by use of an oxygenator. In addition to the reflex systemic resistance changes, the influences of the cardio-pulmonary baroreflexes on systemic venous return and on left atrial contractility will be appraised using the same fundamental preparation modified to record caval flows during constant arterial perfusion rates, and to record atrial dP/dt from an isovolumetric atrial preparation.